Uppercase extensions like .JPG are normal and come from how many cameras store files on their memory cards. These cards often use a simple file-naming system that traditionally writes short filenames in uppercase.

Changing .JPG to .jpg only renames the file; it does not alter the image data inside the file. So it will not reduce quality, recompress the photo, or change the picture content.

In other words:

• .JPG and .jpg are the same file type
• Renaming the extension changes metadata in the filesystem, not the photo itself
• Image quality is unaffected

As long as you keep a valid JPEG extension, your Mac and photo apps should treat the file the same way.